Thanks Nicolas. After some further browsing and thinking I arrived
on the solution.
First, I tried this:
ST_LineMerge( ST_Collect(ST_AddPoint(G1,ST_StartPoint(G2)),G2)) );
By appending the first point of G2 LINESTRING to the end of G1 LINESTRING,
the ST_LineMerge now works. Except, it only works up to and including 3D.
I'm using the 4th dimesion M to store a timestamp with my XYZ points,
and this gets stripped off by ST_LineMerge and returns 3D.
Then I tried:
ST_MakeLine(G1,G2)
and it works! Very embarassing. The key is this comment in
ST_MakeLine documentation:
"Availability: 2.0.0 - Support for linestring input elements was introduced"
This explains why older posts on the net didn't show this obvious solution.
Probably would be helpful to add a simple example to the docs with
two linestrings as input to ST_MakeLine to emphasize the functionality.
Nicolas Ribot wrote:
Hi Andrew,
This (long) thread may give you some hints on how to order and then
merge lines together:
"Closing polylines", 11 May 2012.
Nicolas
On 19 October 2012 00:27, Andrew Ukrainec <aukrai...@accipiterradar.com> wrote:
I've been trying to append a new LINESTRING to an existing
LINESTRING in a table, and this seems to be an impossible
task. I've searched the net for solutions, and tried all
possible obvious approaches:
ST_LineMerge(ST_Collect(geom1,geom2))
ST_LineMerge(ST_Union(geom1,geom2))
...
I wrote a function to use the aggregate version of the call
ST_LineMerge(ST_Collect(geom))
I keep getting a MULTILINESTRING() at the output rather
than a single LINESTRING().
If I run this example adapted from an example on
the OSGeo site:
SELECT ST_AsText(ST_LineMerge(ST_Collect(the_geom)))
FROM (SELECT 'LINESTRING(0 0, 0 1)'::geometry the_geom
UNION ALL
SELECT 'LINESTRING(1 0, 1 1)'::geometry the_geom
UNION ALL
SELECT 'LINESTRING(0 0, 1 0)'::geometry the_geom
UNION ALL
SELECT 'LINESTRING(1 1, 0 1)'::geometry the_geom) as a;
I get a single LINESTRING that I hoped for,
LINESTRING(0 0, 1 0, 1 1, 0 1, 0 0)
but the answer is completely counter intuitive to me.
I expected the individual LINESTRINGs to be simply appended
in order. If I adjust the data in the first LINESTRING so
that '0 1' point is now '1 0':
SELECT ST_AsText(ST_LineMerge(ST_Collect(the_geom)))
FROM (SELECT 'LINESTRING(0 0, 1 0)'::geometry the_geom
UNION ALL
SELECT 'LINESTRING(1 0, 1 1)'::geometry the_geom
UNION ALL
SELECT 'LINESTRING(0 0,1 0)'::geometry the_geom
UNION ALL
SELECT 'LINESTRING(1 1, 0 1)'::geometry the_geom) as a;
then again the operation fails and I get MULTILINESTRING()
at the output.
MULTILINESTRING((1 0,1 1,0 1),(1 0,0 0,1 0))
The PostGIS documentation only contains the mysterious
statement in the Example section:
"--If can't be merged - original MULTILINESTRING is returned"
I can accept that LINEMERGE() does not work the way I imagined
it would, but then there is no explanation of how it does work
in the documentation. In the case that one wants to glue
two LINESTRING() together into a single LINESTRING() preserving
the order of the points, can anyone suggest how this can
be done efficiently?
Thanks in advance.
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--
Dr. Andrew Ukrainec
Principal Engineer e-mail: aukrai...@accipiterradar.com
Accipiter Radar Technologies Inc. http://www.accipiterradar.com
Niagara, Ontario Tel: (905) 228-6888 Fax: (905)892-2249
Orchard Park, New York Tel: (716) 508-4432 Fax: (888)393-6421
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